Astronomy SparkChart - SparkNotes

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SPARKCHARTSTM 

SPARKCHARTS TM 

ASTRONOMY SPARK 

OVERVIEW 

LIGHT 

Wave description: The periodic oscillation of electric and 

magnetic fields in space. Characterized by the following: 

1. Frequency of the oscillation, ν, measured in cycles per 

second (hertz). 

2. Wavelength of the oscillation, λ, measured in meters. 

This is the distance from one peak to the next. 

• Wave equation: All light travels at a finite speed, 

c = 3 × 10 8 meters per second. This results in an 

inverse relationship between the frequency and 

wavelength. 

• Mathematically: c = λ × ν. 

• Thus, light with a high frequency has a short 

wavelength, and vice versa. 

Particle description: A stream of photons, individual 

particles of light that each carry a specific amount of energy, 

which is directly proportional to the frequency of the light. 

• Mathematically, Planck’s Law: E = hν 

E, energy (Joules, J) 

ν, frequency (Hertz, hz) 

h = 6.63 × 10 −34 , Planck’s constant (J × s) 

• Light with a high frequency (short wavelength) is 

also very energetic. 

Electromagnetic spectrum: The collection of all frequencies 

of light. 

• Includes (in order of increasing energy) radio, 

infrared, visible, ultraviolet, x-ray, and gamma ray 

frequencies. 

• Different physical processes in the universe emit 

radiation at different frequencies, so each frequency 

band probes different phenomena in the universe. 

Light quantities: 

• Energy: The capacity to cause change (Joules, J) 

• Power: Energy emitted per unit of time (J/s= Watts, 

W=J s −1 ) 

• Luminosity: Light energy emitted per unit time 

(power from a star) (J s −1 =Watts, W) 

• Flux: Energy emitted per unit time per unit area 

(J s −1 m −2 = Wm −2 ) 

Mathematically, F = L 

4πD 2 

F , flux from surface of a spherical object (Wm −2 ) 

L, luminosity of object (W) 

D, distance to object (m) 

Spectroscopy: The technique astronomers use to separate 

light into its intensity at different wavelengths or spectrum. 

Components: 

1. Continuum: The smooth part of the spectrum (see 

Figure 1). Most objects emit light at all frequencies, but 

the shape of the spectrum depends on the physical 

process that produces the light. 

• Blackbody: A dense object that reflects no light, and 

thus emits light only because of the thermal motion 

of its atoms, measured by its temperature. 

• Most objects produce their own continuum 

approximately as a blackbody (e.g., the Sun, an 

incandescent light bulb, and the human body). 

INTENSITY 

bluer 

T1 

This downloadable PDF copyright © 2004 by SparkNotes LLC. 

T2 

T1 > > 

T2 T3 

T3 

WAVELENGTH 

redder 

Figure 1 

Blackbody curves 

for different 

temperatures. The 

emission from a 

hotter object will 

peak at shorter 

wavelengths. 

• The shape of the curve depends only on 

temperature. A hot object emits more light at 

higher frequencies (higher energies) than a cool 

object (e.g., hot stars appear blue, cool stars 

appear red). 

• Wien’s Law for a blackbody: λmax = 3×10−3 

T 

• λmax, wavelength of maximum intensity (m) 

• T , temperature of blackbody (Kelvins, K) 

• Stefan-Boltzmann Law for a blackbody: F = σT 4 

F , energy flux from surface of blackbody (W m−2 ) 

σ = 5.67 × 10−8 (W m−2K−4 ), Stefan- 

Boltzmann constant 

T , temperature of blackbody (K) 

• This flux is equal to the area under the curve of 

intensity versus wavelength for a blackbody. 

2. Atomic lines: According to quantum mechanics, 

electrons bound to an atom can only have particular 

values of energy; they are unique to that element. 

Absorption or emission of a photon of light by the atom 

occurs when the energy of that photon matches the 

difference between two of these energy levels. 

• Absorption lines: Narrow, dark regions in a spectrum 

produced when an electron uses up a photon to jump 

to a higher energy level in an atom. 

• Emission lines: Narrow, bright regions in a spectrum 

produced when an electron spontaneously drops to a 

lower energy level in an atom. 

Doppler shift: The difference between the wavelength at 

which light is observed and the wavelength at which it was 

originally emitted due to the motion of the emitter relative 

to the observer. 

• Mathematically (for objects moving much slower 

than the speed of light): z = λobs−λem v = λem c 

z, redshift (dimensionless) 

λobs, observed wavelength (any length unit, usually 

nanometers, nm (= 10−9 m) or angstroms, ˚A 

(= 10−10 m)) 

λem, wavelength emitted by the source (same length 

unit) 

v, velocity of moving source (m/s) 

c, speed of light (m/s) 

• Note: 

z > 0: Source moving away, shift to longer 

wavelength (redder) 

z < 0: Source moving toward, shift to shorter 

wavelength (bluer) 

MOTION 

Kepler’s Laws of Planetary Motion 

1. Planets move in elliptical orbits with the Sun at one 

focus of the ellipse (see Figure 2). 

• For a circular orbit, semimajor axis = radius. 

• Implication: Orbits are not perfect circles, but are 

slightly elongated. 

2. Pick an interval of time (e.g., a month). In that amount 

of time, the line connecting a planet to the Sun will 

sweep over the same area, regardless of where it is on its 

elliptical orbit. 

• Implication: Planets move faster when they are 

closer to the sun (see Figure 2). 

ELLIPTICAL ORBIT 

C 

D 

Sun 

Empty focus 

3. The square of the period of revolution (P ) of a planet 

is directly proportional to the cube of the semimajor 

axis (a) of the elliptical orbit. 

B 

A 

Figure 2: 

Kepler’s Second 

Law 

The two shaded 

areas are equal. 

According to the 

second law, equal 

time passes 

between A and B 

and also C and D. 

Thus, the planet 

travels faster 

between C and D. 

CHARTS TM 

• Implication: Planets farther from the Sun take a 

longer time to go around the Sun. 

• Mathematically: P 2 = 4π2 

G(M+m) a3 

P , period (= time planet takes to complete one orbit) 

(seconds, s) 

a, semimajor axis (meters, m) 

M , mass of body being orbited (e.g., the Sun) 

(kilograms, kg) 

m, mass of the orbiting body (e.g., the planet) (kg) 

G = 6.67 × 10−11 , universal gravitation constant 

( N×m2 

kg2 ) 

• If M >> m, m can be ignored 

2 

Pplanet • Mathematically, 

P 2 

Earth 

= a3 

planet 

a 3 

Earth 

P , period (PEarth = 1 in years) 

a, semimajor axis (aEarth = 1 in AU, see Units) 

• More convenient formula when comparing 

another planet to Earth. 

Newton’s Law of Universal Gravitation: All objects in the 

universe attract all other objects with a force dependent 

upon the mass of the two objects and the distance between 

them. 

• Mathematically: F = GM1M2 

R2 F , force (Newtons, N) 

R, distance (m) 

M1, M2, mass of bodies (kg) 

G, universal gravitation constant ( N×m2 

kg2 ) 

Circular velocity: Orbital velocity (vcirc) when one mass 

orbits another in a circle. � 

GM 

• Mathematically: vcirc = R 

M , mass of body being orbited (kg) 

R, radius of orbit (m) 


2 ) 

• Note: vcirc is independent of the mass of the orbiting 

body. 

Escape velocity: Velocity (vesc) needed to completely 

escape from the gravitational pull of another object. 

� 

2GM 

• Mathematically: vesc = R 

M , mass of body being escaped from (kg) 

R, radius of body being escaped from (m) 


kg2 ) 

• Note: vesc is independent of the mass of the escaping 

body. 

UNITS 

We cannot describe stars and galaxies using human scale units. 

ASTRONOMICAL SCALE AND ITS EQUIVALENTS 

Quantity Human Scale Astronomical Scale 

Distance Meter, m Solar radius, R�, 

Kilometer, km (= 7 × 10 5 km) 

SPARKCHARTS Astronomy page 1 of 6 

kg 

Astronomical unit, AU 

(= 1.5 × 10 8 km, 

distance from Earth to Sun) 

Light year, ly 

(= 9.5 × 10 12 km, 

distance light travels in 

one year) 

Parsec, pc 

(= 3.1 × 10 13 km = 3.26 ly) 

Mass Gram, g Earth mass, M⊕ 

Kilogram, kg (= 6 × 10 24 kg) 

Solar mass, M� 

(= 2 × 10 30 kg) 

Time Second, s Gigayear, Gyr 

Year, yr (= 10 9 yr = 1 billion yr) 

Luminosity Watt, W Solar luminosity, L� 

(light power Horsepower, hp (= 3.9 × 10 26 W) 

output) (= 746 W) 

“IT I 

COM 

THE 

THE 

EAR 

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thick lay 

• T 

s 

Motions 

• R 

e 

• 

• 

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3 

Seasons 

• W 

w 

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• A 

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s 

• N 

to 

c 

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S 

Winter 

Hemis 

Figure 

The til 

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Magnet 

charged 

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p 

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possibly 

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ther from the Sun take a 

the Sun. 

4π 2 

(M+m) a3 

takes to complete one orbit) 

s, m) 

g orbited (e.g., the Sun) 

ody (e.g., the planet) (kg) 

versal gravitation constant 

e ignored 

= a3 

planet 

a3 Earth 

n years) 

h = 1 in AU, see Units) 

rmula when comparing 

th. 

vitation: All objects in the 

ts with a force dependent 

s and the distance between 

1M2 

R 2 

(kg) 

constant ( N×m2 

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ity (vcirc) when one mass 

� 

GM 

R 

rbited (kg) 


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t of the mass of the orbiting 

sc) needed to completely 

ll of another object. 

� 

2GM 

R 

scaped from (kg) 

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xies using human scale units. 

EQUIVALENTS 

tronomical Scale 

lar radius, R�, 

7 × 10 5 km) 

tronomical unit, AU 

1.5 × 10 8 km, 

tance from Earth to Sun) 

ht year, ly 

9.5 × 10 12 km, 

tance light travels in 

e year) 

rsec, pc 

3.1 × 10 13 km = 3.26 ly) 

rth mass, M⊕ 

6 × 10 24 kg) 

lar mass, M� 

2 × 10 30 kg) 

ayear, Gyr 

10 9 yr = 1 billion yr) 

lar luminosity, L� 

(= 3.9 × 10 26 W) 

“IT IS CLEAR TO EVERYONE THAT ASTRONOMY AT ALL EVENTS 

COMPELS THE SOUL TO LOOK UPWARDS, AND DRAWS IT FROM 

THE THINGS OF THIS WORLD TO THE OTHER.” PLATO 

THE SOLAR SYSTEM 

EARTH-MOON-SUN SYSTEM 

EARTH 

Composition: Three layers; a solid iron and nickel core, a 

thick layer of mantle, and a thin outer crust. 

• The shape of Earth is an oblate spheroid (a slightly 

squished sphere). 

Motions 

• Rotation: Movement of Earth around its axis once 

every 24 hours. 

• At any given time, the Sun lights up half of Earth. 

• Day and night begin as a spot on Earth moves into 

or out of the illuminated half. 

• Revolution: Earth travels around the Sun once every 

365.25 days (one year). 

Seasons: Caused by the fixed tilt of Earth’s axis (see Figure 3). 

• When the North Pole points away from the Sun, it is 

winter in the Northern Hemisphere. This is because 

the rays of the Sun are tilted and do not warm the 

surface efficiently. 

• At the same time, it is summer in the Southern 

Hemisphere. This is because the rays of the Sun 

strike the surface from almost directly overhead. 

• Note: The slight variation in the distance from Earth 

to the Sun, which is caused by Earth’s slightly noncircular 

orbit, is irrelevant to the changes in seasons. 

23.5° 

23.5° 

N N 

S 

Winter in Southern 

Hemisphere 

Figure 3 

The tilt of the Earth’s rotation axis is the cause for the 

seasons. This diagram is not drawn to scale. 

Magnetic field: Generated by the rotational motions of 

charged particles in the liquid part of the core. 

• It emerges from Earth at the North Magnetic Pole 

(slightly offset from Earth’s rotation axis), and 

returns to the South Magnetic Pole (see Figure 4). 

• Magnetic fields interact with moving charged 

particles and cause them to spiral around magnetic 

field lines. This leads to 3 important phenomena: 

1. Van Allen Belts: Charged particles from space get 

trapped in the magnetic field lines of Earth. 

2. Aurorae (Northern and Southern lights): Caused by 

the deexcitation of atoms and molecules that occurs 

when charged particles trapped by the magnetic field 

strike the Earth’s atmosphere near the poles. 

3. Magnetosphere: Extension of the Earth’s magnetic 

field hundreds of Earth radii into space (see Figure 

4). It traps or deflects the constant flow of charged 

particles from the Sun. The Van Allen Belts are the 

inner parts of the magnetosphere. 

Solar Wind 

Van Allen Belts 

S 

Winter in Northern 

Hemisphere 

Magnetosphere field lines 

Figure 4: Earth’s magnetic field and magnetosphere 

THE MOON 

Composition: Low-density crust, a silica-rich mantle, and 

possibly metallic (iron) core. 

• Highlands: The highly cratered and brighter parts of 

Moon’s surface. 


• Maria (from the Latin for “sea”): The smoother and 

darker lowlands on the Moon’s surface. 

• Because maria are less cratered, they are thought to 

have formed later, possibly by volcanic activity. 

Phases: Caused by the relative alignment of Earth, the 

Moon, and the Sun. 

• One half of the Moon is always illuminated by the Sun. 

• The portion of the illuminated half that we see 

determines the shape of that particular phase. 

• Cycle of phases repeats every 29 1 

2 days (see Figure 5). 

D 

A 

New 

Moon 

C 

E 

B 

Waxing 

crescent 

Moon 

LIGHT FROM SUN 

Figure 5: The phases of the moon. Diagram not to scale. 

Eclipses (see Figure 6) 

• Solar: The Moon moves between the Sun and Earth, 

and casts its shadow on Earth. A total solar eclipse 

(the Moon completely covers the Sun) can occur even 

though the Sun’s radius is 375 times the Moon’s 

radius because the Earth-Moon distance is much less 

than the Earth-Sun distance. 

• Lunar: The Moon moves directly behind the line 

between the Sun and Earth, and Earth casts its 

shadow on the Moon. 

SOLAR ECLIPSE 

C 

First 

quarter 

Moon 

B 

D 

Waxing 

gibbous 

Moon 

F 

VIEWS OF THE MOON AS SEEN FROM EARTH 

E 

Full 

Moon 

F 

Waning 

gibbous 

Moon 

G 

Last 

quarter 

Moon 

H 

Waning 

crescent 

Moon 

Earth Moon Sun 

Earth Moon 

Umbra 

• Eclipses do not occur once a month because the plane 

in which the Moon orbits is tilted about 5 degrees 

from the plane of Earth’s orbit around the Sun. 

Tides occur on Earth as a result of the gravitational pull of 

the Sun and the Moon. 

• The gravitational pull of any object gets weaker the 

further you move from the object. Thus, the Moon 

pulls harder on the water on the side of Earth nearer 

to it than on the water on the side of Earth farther 

from it. This creates a bulge in the water on the side 

of the Earth facing the Moon and another on the 

opposite side (see Figure 7). The Earth itself is more 

rigid, so essentially no bulge is created in its crust. 

A 

G 

H 

LUNAR ECLIPSE 

Penumbra 

Figure 6: Lunar and solar eclipses. Diagram not to scale. 

Figure 7: Spring and neap 

tides. Diagram not to scale. 

SPRING TIDE 

Oceans 

Earth 

NEAP TIDE 

Moon Sun 

High tide 

Low tide 

• A place on the Earth’s surface experiences high tide 

when that place faces toward (or away from) the 

Moon. Low tide occurs when the Earth has rotated 

90 degrees from high tide. 

• During a span of approximately 24 hours, every 

location on Earth passes through 2 high tides and 2 

low tides. 

• Spring tides (unrelated to the season): Strongest 

tides; occur when the tidal bulges created by the Sun 

and Moon line up. 

• Neap tides: Weakest tides; occur when the tidal 

bulges created by the Sun and Moon are at right 

angles to each other (see Figure 7). 

Impact theory of origin: A Mars-sized object struck Earth 

off-center, ejecting material that then formed the Moon. 

This theory is currently favored by geological evidence and 

computer simulations. 

PLANETS 

GENERAL TRENDS OF PLANETARY 

SCIENCE 

Active lifetime: The size of a planet determines its active 

lifetime. 

• The internal heat of a planet comes from 

gravitational contraction during the planet’s 

formation. 

• As the internal heat is radiated away into space, 

changes occur in both the internal structure and 

surface features of a planet. 

• When the heat is gone, the planet can no longer 

evolve from the inside, and it is considered dead. 

Atmosphere: The balance between the force of gravity on a 

planet and its average surface temperature determines the 

amount and composition of its atmosphere. 

• If the average velocity of gas molecules (determined 

by surface temperature) is greater than the escape 

speed of the planet (determined from its mass and 

size, see Orbits), then that molecule will not be 

present in the planet’s atmosphere. 

• Lighter molecules like hydrogen and helium are 

harder for a planet to hold onto because they move 

faster than heavy molecules at a given temperature. 

Internal structure: 

• Differentiation: If the internal heat is high enough, 

the materials that make up a planet will melt and the 

heavier components will sink to the center. 

• Average density: Total mass of a planet divided by 

its total volume. 

• High average density implies a mostly rocky planet. 

• Low average density implies a mostly gaseous planet. 

Surface features: Four main processes mold the surface of 

a planet: 

1. Cratering: Pits in the crust of planets form because of 

impacts with other solar system bodies. 

2. Erosion: Water flows and wind (if an atmosphere is 

present) wear away a planet’s surface features. 

3. Volcanism: Hot rock and other material rise to the 

surface of a planet. 

4. Plate tectonics: A layer of crust is broken into plates and 

rides on a lower layer of softened rock that is heated by 

natural radioactivity. 

• This theory explains earthquakes and volcanos by 

inferring that they are the result of plates being 

pushed together or driven apart. 

• Plate tectonics occurs only on Earth. 

TERRESTRIAL PLANETS 

Mercury, Venus, Earth (and its moon), and Mars. 

1. Atmosphere 

• Earth and Venus: Bigger planets can hold onto heavy 

molecules (such as nitrogen, carbon dioxide, and 

oxygen), but not hydrogen because the planets’ 

surface temperatures are too high. 


ASTRON 

THE S 

• 

• Mo 

bec 

• Ma 

COMPARI 

Planet 

Mercury 

Venus 

Earth 

Mars 

Moon 

2. Interna 

differe 

densiti 

3. Surfac 

• Ear 

vol 

con 

awa 

• Mo 

atm 

hap 

act 

• Ma 

like 

act 

JOVIAN 

Jupiter, S 

similaritie 

largest of 

1. Atmos 

• Jov 

and 

• The 

rot 

• The 

dee 

• 

• 

COMPARIS 

Planet 

Jupiter 

Saturn 

Uranus 

Neptune 

Pluto 

Me 

PICTURES 

2. Interna 

constu

EVENTS 

IT FROM 

PLATO 

’s surface experiences high tide 

es toward (or away from) the 

rs when the Earth has rotated 

tide. 

pproximately 24 hours, every 

sses through 2 high tides and 2 

ted to the season): Strongest 

tidal bulges created by the Sun 

st tides; occur when the tidal 

e Sun and Moon are at right 

(see Figure 7). 

Mars-sized object struck Earth 

l that then formed the Moon. 

ored by geological evidence and 

OF PLANETARY 

a planet determines its active 

of a planet comes from 

action during the planet’s 

t is radiated away into space, 

th the internal structure and 

planet. 

one, the planet can no longer 

e, and it is considered dead. 

etween the force of gravity on a 

ce temperature determines the 

its atmosphere. 

y of gas molecules (determined 

ure) is greater than the escape 

(determined from its mass and 

en that molecule will not be 

’s atmosphere. 

ike hydrogen and helium are 

o hold onto because they move 

lecules at a given temperature. 

e internal heat is high enough, 

ke up a planet will melt and the 

will sink to the center. 

tal mass of a planet divided by 

ity implies a mostly rocky planet. 

y implies a mostly gaseous planet. 

n processes mold the surface of 

rust of planets form because of 

system bodies. 

nd wind (if an atmosphere is 

anet’s surface features. 

nd other material rise to the 

f crust is broken into plates and 

softened rock that is heated by 

earthquakes and volcanos by 

are the result of plates being 

riven apart. 

s only on Earth. 

NETS 

its moon), and Mars. 

ger planets can hold onto heavy 

nitrogen, carbon dioxide, and 

ydrogen because the planets’ 

are too high. 

ASTRONOMY 

THE SOLAR SYSTEM (CONTINUED) 

Mercury Venus Earth Mars 

PICTURES NOT TO SCALE 

• Greenhouse effect: Radiation incident on the 

surface of Venus is re-radiated at longer 

wavelengths that cannot get back out through 

the atmosphere. This trapped radiation heats 

the surface to very high temperatures. The same 

thing happens inside a closed car on a hot day. 

• Moon and Mercury: Have almost no atmosphere 

because of their low surface gravity. 

• Mars: Has a thin atmosphere of carbon dioxide. 

COMPARISON OF THE TERRESTRIAL PLANETS 

Planet Diameter Mass Density Surface Pressure 

Earth = 1 Earth = 1 Water = 1 Earth = 1 

Mercury 0.38 0.055 5.4 10 -15 

Venus 0.95 0.82 5.2 100 

Earth 1.0 1.0 5.5 1.0 

Mars 0.53 0.11 3.9 0.01 

Moon 0.27 0.012 3.3 10 -15 

2. Internal structure: All terrestrial planets have undergone 

differentiation (see Internal structure). They have high 

densities because of their rocky interiors. 

3. Surface features 

• Earth and Venus: The most active planets. Erosion, 

volcanic activity, and plate tectonics (on Earth) 

constantly renew their surfaces, rapidly wiping 

away evidence of cratering. 

• Moon and Mercury: Because they lack an 

atmosphere, they retain their craters. Cratering 

happened long ago, showing that no other surface 

activity has taken place for a long time. 

• Mars: Has huge dormant volcanoes and riverbedlike 

features that imply there was some volcanic 

activity and liquid water flow in its recent past. 

JOVIAN PLANETS 

Jupiter, Saturn, Uranus, and Neptune. Because of their 

similarities in composition, they are named after the 

largest of the group, Jupiter. 

1. Atmosphere 

• Jovian planets are made of gas in their outer layers 

and have no solid surface. 

• They exhibit differential rotation (gas at the equator 

rotates faster than gas at the poles). 

• Their hot interiors drive hot, dense material from 

deep in the atmosphere up to the cloud tops. 

• This results in a banded structure, which we can 

see most clearly in Jupiter and Saturn, and less 

so in Uranus and Neptune. 

• Small amounts of methane in the atmospheres 

of Uranus and Neptune give them a greenish 

and bluish color, respectively. 

COMPARISON OF THE JOVIAN PLANETS 

Planet Diameter Mass Bulk Density 

Earth = 1 Earth = 1 Water = 1 

Jupiter 11.0 318 1.3 

Saturn 9.5 95 0.7 

Uranus 4.1 15 1.2 

Neptune 3.9 17 1.7 

Pluto 0.17 0.002 2.1 

2. Internal structure: Two characteristics allow us to 

constuct models of Jovian planets: 


a. Their sunlike density. 

b. Their ability to radiate more energy into space than 

they absorb from the Sun. 

• The models show that Jovian planets are active, 

differentiated worlds composed of: 

• A molecular hydrogen atmosphere 

• A liquid hydrogen (and metallic hydrogen in 

Jupiter and Saturn) interior 

• A (possibly) rocky core 

3. Surface features: 

• Jovian worlds do not have a solid surface, so 

evidence of cratering, erosion, and volcanism are 

not present. 

• Despite this, there are some stable surface features, 

including the Great Red Spot of Jupiter and the 

Great Dark Spot of Neptune, atmospheric storms of 

tremendous intensity that will last for many years. 

4. Moons: All Jovian planets have a large system of moons. 

• Tidal locking: Tidal forces have acted over time so 

that the same half of a moon points toward the parent 

planet at all times (as seen with Earth’s moon). 

5. Rings: All Jovian planets have rings. 

• Composition: Made up of many individual particles 

averaging one meter in size, all orbiting their parent 

planet according to Kepler’s Laws (see Orbits) in a disk 

about a kilometer thick. There are two types of particles: 

a. Icy: Seen as bright rings; found around Saturn. 

b. Rocky: Seen as dark rings; found around 

Jupiter, Neptune, Uranus. 

• Shepherd moons: A pair of small moons, one 

interior to the ring and one exterior to the ring 

which help to maintain the stability of the ring. The 

gravitational tugs of the shepherd moons act to 

herd the ring particles into the same orbit. 

• Pluto: The outermost planet and its moon, Charon, do 

not fit either of the above categories. Instead, they can 

be thought of as the largest icy bodies in the Kuiper Belt 

(see Comets). 

EXTRA-SOLAR PLANETS 

• “Hot Jupiter”: One of dozens of planets, with masses 

close to that of Jupiter, found orbiting very close to 

another star. 

• Migration: The current theory states that hot Jupiters 

formed further from their star and then migrated 

inward by some uncertain mechanism. 

Detection techniques: 

1. Doppler shift: A massive orbiting planet causes the 

position of its parent star to wobble. We can determine 

some orbital properties of the planet by watching the 

spectrum of the star shift back and forth because of 

this wobble. 

2. Transits: If a planet passes in front of its parent star, we 

see a dip in the intensity of the light from that star. 

From the shape of this dip, we can determine some 

properties of the planet, including its orbital period 

and size relative to its parent star. 

EXTRAS OF THE SOLAR 

SYSTEM 

COMETS 

Composition: Comets consist of four parts: 

a. Nucleus: A dirty snowball a few kilometers wide consisting 

of water and other organic ices mixed with dust. 

b. Coma: Region of gas and dust thrown off from the nucleus, 

measuring up to a million kilometers in diameter. 

Jupiter Saturn Uranus Neptune 

Uranus photo courtesy of NSSDC. Venus © DigitalVision 

Mercury, Earth, Mars, Jupiter, Saturn, and Neptune © PhotoDisc, Inc. 

c. Gas tail: Ions blown off the nucleus by the solar wind 

(see Solar Activity). 

d. Dust tail: Particles released from the melting ice that curve 

behind the gas tail; up to 150 million kilometers long. 

• Note: Tails are only present when a comet is close 

to the Sun. 

Origin: 

1. Kuiper belt: A collection of comet nuclei that orbit just 

beyond the orbit of Neptune. 

• Most comets with orbits of less than 200 years 

originate here. 

2. Oort Cloud: A collection of comet nuclei in a shell 

about 1,000 times as far out as Pluto’s orbit. 

• Most comets with orbits of longer than 200 years 

originate here. 

METEORITES 

• Chunks of matter up to tens of meters across, left 

behind by passing comets, may burn up as they pass 

into Earth’s atmosphere. They have different names 

depending on where they are relative to the 

atmosphere: 

1. Meteoroid: A chunk outside the atmosphere. 

2. Meteor (shooting star): The flash of light a chunk 

makes as it burns up in the atmosphere. 

3. Meteorite: A chunk that makes it to the ground. 

• Composition: There are types of meteorite: iron, stony, 

and stony-iron. 

ASTEROIDS 

• Minor planets that orbit mostly in a gap between the 

orbits of Mars and Jupiter. 

• Kirkwood gaps: Empty orbits in the asteroid belt with 

an orbital period of exactly 1 1 

2 or 3 that of Jupiter. They 

are empty due to orbital resonance. 

• Orbital resonance: The repeated tugging between 

two bodies on orbits with whole number orbital 

period ratios. 

• An asteroid and Jupiter both orbit the Sun, with 

the asteroid orbiting faster (from Kepler’s Third 

Law, see Kepler’s Laws) 

• Because of their period ratios, they are always closest 

to each other at the same point on their orbits. 

• The repeated gravitational tug the asteroid feels 

from Jupiter acts to tug it out of that orbit into one 

that does not have a perfect orbital period ratio. 

• This phenomenon also occurs for moons and 

rings orbiting a planet. 

FORMATION OF THE SOLAR 

SYSTEM 

Any formation scenario must explain the following 

evidence: 

• All planets orbit the Sun in nearly the same plane, 

in nearly circular orbits. 

• All planets travel around the sun in the same direction, 

which is also the direction of the Sun’s rotation. 

• Smaller, rocky planets orbit near the Sun, while larger, 

gaseous planets orbit further away from the Sun. 

• In the last ten years, we have found many nearby stars 

with planetary systems. Therefore, planet formation 

must be almost as common as star formation. 

Nebular theory: The most widely accepted theory of solar 

system formation (see Figure 8). 

a. About 5 billion years ago, a cloud of interstellar dust 

began to collapse. The trigger for this collapse is still 

unclear. 

CONTINUED ON OTHER SIDE 

SPARKCHARTS Astronomy page 3 of 6

E 

r 

Astronomy 4.x.1 3/21/03 10:49 AM Page 2 

THE SOLAR SYSTEM (CONTINUED) 

b. As the cloud of dust continued to collapse under its 

own gravity, the rotational speed of the particles 

increased because of conservation of angular 

momentum—the reason why an ice skater spins 

faster when his or her arms are pulled in—and the 

cloud became a flattened disk. 

c. Energy from the gravitational collapse caused the 

temperature in the center of the disk to rise until 

fusion started in the protosun. Micrometer-sized bits 

of dust began to stick together, and after about 

10,000 years, they reached a size of tens of kilometers 

across and were called planetesimals. 

d. Over tens of millions of years, the planetesimals collided 

and combined due to gravity, eventually forming planets. 

STELLAR ASTRONOMY 

OUR SUN 

STRUCTURE 

Solar interior 

• Core: Inner 10% of the Sun’s radius. The nuclear 

fusion that powers the Sun takes place in the core 

at a temperature of 15 million Kelvins. 

• Radiative zone: Layer of the Sun directly above 

the core. Energy is transported outward by 

radiation (the movement of photons). 

• Convective zone: Layer of the Sun above the 

radiative zone. Energy is transported outward by 

convection (hot gas rises and cooler gas falls). 

Solar atmosphere 

• Photosphere: Layer of the Sun we can see (the 

Sun’s surface). 

Temperature: 5800 K 

Composition: 74% hydrogen, 25% helium, and 1% 

all other elements (same as the rest of the sun). 

Granulation: Lighter and darker regions about 

1,000 km across, which cover the photosphere 

with a pattern that changes on average every 10 

minutes. They are created by convection that 

brings hot material to the Sun’s surface and 

pulls cooler material below the surface. 

• Chromosphere: Layer of the Sun above the 

photosphere that shows a pinkish glow during a 

total solar eclipse. 

Temperature: Rises from 4,200 K to 1 million K; 

due to radiation from photosphere as well as 

from magnetic fields extending up from the 

photosphere into the chromosphere. 

Thickness: 2,000 km 

• Corona: Layer above the chromosphere that is 

only visible during a total solar eclipse. 

Temperature: Approximately 2 million K; 

mechanism which generates this intense heat 

is unclear, but is related to magnetic activity. 

Composition: Small number of highly ionized 

atoms (low density) moving at high speeds 

(high temperature). 

Thickness: Extends from the top of the chromosphere 

out into the rest of the solar system. 

MAGNETIC FIELD 

Magnetic dynamo model: Description of how the Sun’s 

magnetic field changes over time. Two motions bring this 

about: 

1. Differential rotation: Gas at the equator (25-day period) 

rotates faster than gas at the poles (31-day period). 

• The Sun’s magnetic field lines are locked into the 

material just beneath the photosphere. Thus, 

when material at the surface near the equator gets 

pulled around more quickly than the material near 

the poles, the magnetic field lines get wound 

around the Sun (see Figure 9). 

2. Convection at the Sun’s surface: Hot material below 

the photosphere rises to the surface, as in granulation 

(see Photosphere). 

• Magnetic field lines locked into this material are 

thus brought up to the surface of the Sun. If the 

lines are wound tightly enough by differential 

rotation, they will kink and pop out of the 

photosphere, causing small regions of magnetic 

field that are thousands of times stronger than the 

Sun’s average magnetic field (see Figure 9). 


• The planetesimals that did not form planets are 

thought to be the comets and asteroids in the solar 

system today. 

• Rocky planets formed near the Sun because the 

heat vaporized most icy components. Most 

gaseous components escape these planets because 

the surface temperature is too high and the force 

of gravity is too low (see Atmosphere, under Trends 

of Planetary Science). 

• Gas giant planets formed far from the Sun because 

the cooler gas could not escape their larger 

gravitational pull. 

Figure 9: Magnetic dynamo model 

SOLAR ACTIVITY 

• Most solar activity is thought to be related to the 

magnetic field of the Sun. 

• Sunspots: Temporary areas of cooler and darker gas 

that appear in groups on the photosphere. 

• Temperature: 4,200 K (less than 5,800 K average 

photosphere temperature). 

• Cause: Regions of strong magnetic field restrain 

the convective motion that brings hotter gas to the 

surface. Thus, sunspots are cooler. 

• Sunspot cycle: 11-year cycle, beginning with just a 

few sunspots near the poles that are replaced by more 

sunspots near the equator as the cycle continues. 

• Solar flares: Violent solar storms associated with 

regions of sunspots (and thus strong magnetic fields). 

• Temperature: Around 5 million K 

• Duration: About 20 minutes 

• Effect on Earth: Light reaches Earth in minutes, 

but energetic charged particles from the flare 

arrive on Earth days later. If the particles 

penetrate Earth’s magnetosphere, they may 

disrupt radio communication or disable satellites 

orbiting Earth (see Magnetic Field under Earth). 

• Solar prominences: Loops of gas that can rise tens of 

thousands of kilometers above the surface of the Sun. 

• Temperature: Around 10,000 K (this is how they 

are distinguished from solar flares). 

• Solar wind: Extension of the charged particles of the 

corona out into the solar system. 

• Temperature: Around 2 million K. 

STARS AS SUNS 

WHAT ARE THEY? 

• Star: Enormous, self-sustaining nuclear reactor made 

of mostly hydrogen gas; only known structures in the 

universe capable of taking the simple atoms present at 

the beginning of the universe and fusing them into the 

heavier elements needed for life. 

• Hydrostatic Equilibrium: A balance in the battle of 

gravity versus pressure at every point inside a star; a star 

will not expand, contract, or shift its internal structure. 

• Pressure: Outward force resulting from processes 

of energy generation (usually heat from fusion) or 

quantum mechanical effects (called degeneracy, 

see White Dwarfs). 

• Gravity: Inward force that attracts each bit of gas 

toward the center of the star. 

• Note: Major changes in the star during its lifetime 

occur when one of these forces temporarily wins 

out over the other, but the star always settles to a 

new state of equilibrium. 

A B 

Protosun 

Planetesimals 

Planets 

HOW DO THEY WORK? 

• Fusion: The combination of lighter atomic nuclei into 

heavier atomic nuclei. Fusion can only take place at 

very high temperatures (i.e., fast moving particles) 

and high pressures (i.e., tightly packed particles) 

because electrostatic repulsion between two positive 

nuclei must be overcome before the strong nuclear 

force causes the nuclei to combine. 

• Hydrogen fusion: There are two processes: 

1. Proton-proton (PP) chain: Four hydrogen nuclei fuse 

into one alpha particle and release high energy 

photons and neutrinos. 

• Alpha particle = helium nucleus 

• Requires chain of 3 separate nuclear reactions to 

go from initial reactants to final products. 

• Photons take about a million years to reach the 

surface of the star. 

• Occurs mostly in low mass stars (1.5 M� or less). 

2. Carbon-nitrogen-oxygen (CNO) cycle: Net reaction is 

the same as the PP chain: four hydrogen nuclei create 

one alpha particle. 

• Intermediate reaction steps involve isotopes of 

carbon, nitrogen, and oxygen as catalysts (they 

participate in reactions, but do not get used up). 

• More efficient than PP chain = higher yield of 

energy for same products. 

• Occurs mostly in higher mass stars (greater than 

1.5 M�). 

• Helium fusion (Triple alpha process): Three alpha 

particles fuse into one carbon nucleus. 

• Occurs after all hydrogen in the core has been 

converted to helium. 

• Requires much higher temperatures, up to 100 

million Kelvin. 

• Nucleosynthesis: Heavier nuclei are created from the 

products of previous fusing stages. 

• Begins with the production of oxygen from carbon 

after all helium has been fused into carbon. 

• Iron is the most massive nucleus that can be 

produced by fusion and still release energy. 

• All heavier elements in the universe are formed in 

the unusual processes that accompany a 

supernova (see Stellar Endpoints). 

WHY ARE THEY HERE? 

Star Formation: The process of star formation is believed 

to be similar to the formation of our own Sun (see 

Formation of the Solar System). 

• Site of formation: Giant Molecular Clouds 

(GMCs), a cloud of mostly cool molecular 

hydrogen with a temperature of 20 K and a total 

mass of around 1 million solar masses. 

• Process: 

1. Jeans Mass: If a region of a GMC of a particular 

size contains more than this amount of matter, the 

region will begin to collapse under its own gravity. 

2. Fragmentation: As a clump of material collapses 

and becomes more dense, smaller regions begin to 

collapse inside the larger clump. 

3. Protostar: The fragmented regions heat up due to 

the release of gravitational potential energy as 

they collapse (gravity wins over pressure) until the 

temperature and pressure become high enough to 

start hydrogen fusion. 


Sun 

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Figure 8: Formation of the solar system from solar nebula 

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STELLAR ASTRONOMY (CONTINUED) 

Hertzsprung-Russell (H-R) diagram: Plot of luminosity 

(total power output) versus surface temperature on 

which a star is represented by a point at the position 

matching its current properties (see Figure 10). 

• Main sequence: A strip on the H-R diagram where 

most stars spend the majority of their lives. (see 

Figure 10). 

a. Cool, low mass, dim stars are points at the lower 

right end of the main sequence. 

b. Hot, high mass, bright stars are points at the upper 

left end of the main sequence. 

c. Our Sun’s point is approximately in the middle of 

the main sequence. 

• Evolutionary track: The line formed by the 

movement of the star’s point on the H-R diagram 

over its lifetime; caused by changes in composition 

of the star. 

LUMINOSITY 

8 

bluer 

Figure 10 

Evolution: The mass (primarily) and composition 

(secondarily) of a star determine the shape of the 

evolutionary track a star will follow on the H-R diagram. 

• The mass of a star is fixed for the majority of its life, 

but nuclear fusion changes the internal 

composition. Thus, changes that occur during the 

process of fusion are responsible for changes in the 

star’s position on the H-R diagram over its lifetime. 

• Evolution of low mass stars (less than about 4 solar 

masses): Stages are numbered in Figure 10. 

1. A solar-type star spends 80% of its life on the main 

sequence. 

2. As the star exhausts the supply of hydrogen in its 

core, it increases slightly in luminosity. 

3. When the core runs out of hydrogen, fusion stops 

and the core begins to collapse (gravity is stronger 

than pressure). Gravitational potential energy 

heats the region around the core and a shell of 

hydrogen just outside the core begins fusion. Due 

to some complex stability relationships, as the core 

inside this shell collapses, the rest of the star 

outside the shell expands and therefore cools, 

causing the star to become redder. The star has a 

cooler temperature, but because of the larger 

surface area, it has an overall increase in 

luminosity (total energy output). The star is now a 

red giant. 

4. The core continues collapsing and heating (as the 

outer layers expand) until it is hot enough to begin 


9 

Supernova 

Main sequence 

Helium fusion in the core 

Hydrogen fusion in the core 

H-R diagram showing the main sequence and the 

evolutionary tracks of a low mass star and a high 

mass star 

5 

2 

1 

TEMPERATURE 

redder 

EXTRAGALACTIC ASTRONOMY 

GALAXIES AND COSMOLOGY 

GALAXIES 

Spiral galaxies: The galaxy we inhabit, the Milky Way, is an 

example of a spiral galaxy (see Figure 12). Most spiral 

galaxies are composed of three parts: 

1. Disk: 

• Thin: Diameter is usually 100 times larger than 

thickness. 

6 

3 

7 

4 

the triple alpha process of fusing helium. This 

occurs quickly and is called the helium flash, 

although we cannot observe it, since it happens 

deep in the core of a star. 

5. The core expands and the outer layers contract as 

helium fusion brings stability back to the star. The 

star gets less luminous, but goes to a higher 

temperature. 

6. When the helium in the core is exhausted, a similar 

process of core contraction and outer layer 

expansion occurs—due to helium and hydrogen 

fusion in shells. This is just like the process in step 

3. Once again, the star becomes a red giant. 

7. Helium fusion is very sensitive to temperature, so 

bursts of fusion produce thermonuclear explosions 

in the outer shell called thermal pulses. 

8. A strong stellar wind begins to blow off the tenuous 

outer layers of the star. This process takes about 

1,000 years. The ejected material expands outward 

with a speed of 20 km/s, and forms a bright ring 

called a planetary nebula around the (now 

exposed) hot core of the star. 

9. The hot core of the star does not reach the 

temperatures required to fuse carbon by 

gravitational contraction (carbon is the product of 

helium fusion), so it collapses until gravity is 

balanced by electron degeneracy pressure. This is 

a quantum mechanical effect that results in a 

constant pressure, regardless of temperature. The 

core is now called a white dwarf, and it continues 

to cool until its remaining energy has radiated away 

and it becomes a black dwarf. 

• Evolution of high mass stars (greater than 8 solar 

masses): 

• Massive stars live shorter lives: Essentially, the 

process of evolution is the same as that of a low 

mass star until step 6, but the process occurs 

about 100 times faster. 

• Briefly, the evolution is as follows: hydrogen 

fusion, core collapse and hydrogen shell fusion 

to cause the first expansion into a red giant, 

helium fusion which begins without a helium 

flash, core collapse and helium shell fusion to 

cause the second expansion into a red giant, 

followed by the beginning of thermal pulses. 

• Type II supernova: Although the cutoff depends 

on the star’s composition, a star more massive 

than about 8 solar masses will come to a much 

more violent end. The core can get hot enough 

to fuse carbon and then heavier elements. These 

processes generate energy so quickly that the 

star may blow itself apart. 

STELLAR ENDPOINTS 

The final product of stellar evolution is strongly 

dependent on the mass of the star after it has shed its 

outer layers. The following are final products: 

1. White dwarf: For stars with 0.1 to 1.4 solar masses at 

the end of evolution, the electron degeneracy 

pressure of the material is enough to counteract 

gravity and hold the object in hydrostatic equilibrium. 

No fusion occurs in a white dwarf. 

• Nova: If a white dwarf has a companion red giant 

star, fresh hydrogen from the loosely bound outer 

layers of the red giant fall onto the surface of the 

white dwarf, igniting a brief thermonuclear 

explosion until the new hydrogen has been fused. 

• Dusty: Home of the GMCs, the places where 

starbirth occurs (see Star Formation). Thus, the 

stars in the disk are typically younger. 

• Spiral arms: Two or more twisted regions of 

increased density seen in most disks; they are not 

unchanging structures. Thought to be density waves. 

• Density waves: Stars are slowed down by 

gravitational effects at certain places along their 

orbits and thus get bunched up like cars in the 

bottleneck of a traffic jam. 

2. Neutron star: If the post-ejection core is greater than 

1.4 solar masses (called the Chandrasekhar limit), the 

electron degeneracy pressure is not enough to balance 

gravity. The resulting gravitational collapse causes 

free electrons to combine with protons to form 

neutrons, and it is neutron degeneracy pressure that 

eventually halts the collapse. No fusion occurs in a 

neutron star. 

• Type I supernova: The explosion that occurs when 

a red giant companion dumps enough mass onto its 

white dwarf companion to push it over the 

Chandrasekhar limit. In contrast to a Nova, there is 

probably nothing left after the explosion. 

• A neutron star could also be the remnant of a type 

II supernova explosion (see Evolution of high mass 

stars). 

• Pulsar: A rapidly rotating neutron star. 

• Lighthouse effect: The magnetic field axis of the 

neutron star is offset from its rotation axis. The 

magnetic field accelerates charged particles that 

then give off radiation in the direction of the 

magnetic poles. As the neutron star rotates, we 

see this light in pulses (see Figure 11). 

Magnetic fields 

Neutron star 

Rotation axis 

Figure 11: The lighthouse model of a pulsar 

Synchrotron radiation 

Charged particles 

3. Black hole: A massive star (greater than 3 solar masses 

after shedding its outer layers) that even neutron 

degeneracy cannot support. 

• The force of gravity at the star’s surface will 

increase to the point where the escape velocity (see 

Orbits above) will be equal to the speed of light. No 

light escapes, so the object appears black. 

• Observed by their gravitational effect on other 

objects (a partner star or surrounding gas), as well 

as by the intense x-ray radiation any ionized 

infalling material emits as it is accelerated toward 

the black hole. 

MAIN SEQUENCE MASS RANGES FOR DIFFERENT 

STELLAR ENDPOINTS 

Mass while on the main sequence Resulting stellar 

(in solar masses) endpoint 

0.1 – 0.5 White dwarf 

0.5 – 8 Planetary nebula, 

leaving a white 

dwarf 

8 – 20 Supernova, 

leaving a neutron 

star or black hole 

Greater than 20 Supernova, 

leaving a black hole 

• Classified according to how tightly their two or more 

spiral arms are wound. 

2. Halo: Spherical region that extends up to ten times 

farther in radius than the luminous disk. 

• Majority of matter in this component does not emit 

light (e.g., is dark matter), but we know it is present 

because of gravitational effects. 

• Globular clusters: groups of (usually older) stars 

that orbit in the halo together. 


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EXTRAGALACTIC ASTRONOMY (CONTINUED) 

Tidal Tail 

Spiral Arm 

Bulge 

Figure 12: M51—The whirlpool galaxy (note: this is also an interacting galaxy) 

3. Bulge: The thickening of stars and gas at the center of a spiral galaxy. 

• Supermassive black hole: The rapid motion of stars and gas very close to the 

galactic center implies the existence of one of these at the center of most spiral 

galaxies, including our own. 

Elliptical galaxies: The other fundamental type of galaxy. 

• Much less dust than spiral galaxies; the absence of dusty GMCs in most ellipticals 

means there is much less star formation than in spiral galaxies. Thus, most 

ellipticals are composed of older stars than their spiral counterparts. 

• Football-shaped and classified according to how squished that football shape is 

(see Figure 13). 

Interacting galaxies: Although individual stars in a galaxy are so distant from one 

another that a collision is unlikely, certain places in the universe have a high density of 

galaxies, so the chance of a galaxy collision is much greater. One theory states that 

elliptical galaxies are the leftovers from galaxy interactions between spiral galaxies. 

• When two spiral galaxies collide, the difference in gravitational force from one side 

of a galaxy to the other, called the tidal force, disrupts the orbits of the stars in the 

galaxy, causing its spiral shape to be distorted. 

• Models predict that streamers of gas and stars, called tidal tails, may be the product 

of such an interaction. 

Distances to galaxies: A majority of extragalactic astronomy involves finding the 

distances to objects. Two of the most useful ways to measure distance are the following: 

1. Standard candles: If we can predict the theoretical luminosity (total power 

generated) of an astrophysical event, then we can determine the distance to that 

phenomenon by comparing the theoretical luminosity to how luminous that event 

appears in the sky, (e.g., if two friends run away from you in the darkness with 

identical flashlights, you know the dimmer flashlight is farther away). Examples: 

• Cepheid variable stars: Unstable, pulsating stars whose period of pulsation is 

related to its luminosity. 

• Supernovae: Explosions that always have the same peak luminosity (see Stellar 

Endpoints). 

2. Hubble’s Law: Objects that are farther from us move away faster. 

• Expansion of the universe discovered by Hubble in 1924. 

• Mathematically: v = H0 × d 

v, velocity of receding galaxy (km/s) 

d, distance to galaxy (megaparsecs, Mpc) 

H0 ≈ 70, Hubble constant (km/s/Mpc) 

• If the spectrum of a galaxy is redshifted (see Doppler shift), we can calculate how 

fast it is moving away and, from the Hubble Law, calculate its distance. 

Galaxy clustering: Galaxies are not uniformly distributed when their three-dimensional 

positions relative to Earth are plotted. Astronomers have found: 

1. Voids: Regions with few galaxies. 

2. Clusters: Regions with many galaxies. 

3. Filaments: Strings of galaxies connecting the voids and clusters (see Figure 14). 

COSMOLOGY 

The study of the nature and evolution of the universe as a whole. Assumptions: 

1. The universality of physics: Physics must be the same everywhere in the universe, 

otherwise we could not describe it. 

2. The universe is homogeneous: Matter and radiation are spread out evenly; any 

clumps (e.g., people, stars, or galaxies) are small compared to the size of the universe. 

3. The universe is isotropic: Space looks the same regardless of what direction you look. 

Big Bang theory: The universe (and thus, all matter and energy in it) was once 

compressed into a hot, dense point at some finite time in the past and has expanded 

ever since. The big bang did not expand into anything, but rather space itself 

expanded. Evidence: 

1. Current expansion of the universe: Today’s expansion implies that all matter and 

energy were once squished to a hot, dense point. 


Disk 

Todd Boroson (NOAO), AURA, NOAO, NSF 

Figure 13: M87—An elliptical galaxy 

2. Abundance of Helium: Theoretical predictions of this quantity based on the 

conditions directly after the Big Bang match the abundance of helium found in the 

oldest stars. 

3. Cosmic Microwave Background Radiation (CMBR): There should be a leftover glow 

from the dense, hot conditions of the early universe. The temperature of this 

“background radiation” has been measured (see Spectroscopy) and found to agree 

exactly with the prediction of a blackbody spectrum at 2.7 K. 

−26 kg 

Critical Density (≈ 1.88 × 10 cm3 ): The future of the universe is determined by 

comparing its actual density to the critical density. There are three possibilities: 

1. Closed universe: Average density is greater than critical density. 

• Eventually, the gravitational attraction between all matter will stop the cosmic 

expansion, reverse it, and the universe will end in a Big Crunch. 

2. Flat universe: Average density is equal to critical density. 

• The gravitational attraction between all matter will slow the cosmic expansion, but 

will take an infinite amount of time to stop it. 

3. Open universe: Average density is less than critical density. 

• The gravitational attraction between all matter will slow the cosmic expansion, but 

expansion will never stop. 

Current density estimates: 

1. Visible matter: Stars and gas that emit light; approximately 0.5% of critical density. 

2. Dark baryons: Materials made of protons, neutrons, and electrons that do not emit 

light; approximately 4% of critical density. 

2. Non-baryonic dark matter: Exotic material that is not the normal matter we are used 

to (protons, neutrons, and electrons), whose existence is proved by its gravitational 

effect; approximately 25% of critical density. 

3. Dark energy: 70% of critical density. 

• Recent observations using type I supernovae as standard candles (see Distances to 

galaxies) have found that the expansion of the universe is not slowing down (as in 

all models mentioned above), but instead speeding up. 

• Dark Energy: A repulsive force introduced by theorists to explain the accelerating 

expansion of the universe. 

• Represented in equations by Λ, the cosmological constant. 

• Its true nature remains one of the most important problems in modern 

cosmology and physics. 

Figure 14: A redshift survey showing voids and clusters in the 

large-scale structure of the universe 

Report errors at: 

www.sparknotes.com/errors 

Writer: Mark A. Hartman 

Editor: Karen Schrier 

Design: Dan O. Williams 

Series Editor: Sarah Friedberg 


AURA, NOAO, NSF 

Graphic courtesy Matthew Colless and the 2dF Galaxy Redshift Survey team 

7 2 0 5 9 3 3 6 3 7 5 6 

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